Convert coordinates from DMS to decimal.R

# Converting coordinates from DMS or DdM formats to decimal
# DMS = "degrees minutes seconds"; DdM = "degrees decimal minutes" 
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
dg2dec <- function(varb, Dg=NA, Min=NA, Sec=NA, SW.Hemisphere="S|W") {
  # Dg=decimal, Min=minutes and Sec=seconds; 
  # NOTE 1 - if the format is "degrees decimal minutes - DdM" (e.g. 40° 26.767′ N) and not 
  # "degrees minutes seconds - DMS" (e.g. 40° 26′ 46″ N), then call the function only with 
  # Dg and Min arguments, like dg2dec(varb, Dg="°", Min="′N"). 
  # Same applies when there is no seconds symbol (e.g. 45°12'7.38).
  # Note that one should not use blank spaces in Dg, Min or Sec arguments (will return NA).
  # For more details on formats see: 
  # https://en.wikipedia.org/wiki/Geographic_coordinate_conversion#Coordinate_format_conversion
  
  # Use paste0("[", Dg, Min, Sec, "]") to build regex [] pattern
  # therefore, strsplit() will split string "varb" by what symbols you give to Dg, Min, Sec
  DMS <- sapply(strsplit(varb, paste0('[', Dg, Min, Sec, ']')), as.numeric)
  
  # DMS is a matrix; first row contains degrees; second - minutes; third - seconds.
  # If the format is "degrees decimal minutes" (e.g. 40° 26.767′ N) and not 
  # "degrees minutes seconds" (e.g. 40° 26′ 46″ N), then the matrix has only two valid rows: 
  # first row contains degrees; the second - minutes;
  # therefore, compute conversion for seconds only if there are more than 2 rows in DMS 
  # and Sec is different from NA (if there are seconds in the DMS format)
  decdg <- abs(DMS[1, ]) + DMS[2, ]/60 + ifelse(dim(DMS)[1] > 2  & !is.na(Sec), DMS[3, ]/3600, 0)
  
  # all cordinates from Southern or Western Hemispheres become negative in their decimal format
  SW <- grepl(pattern = SW.Hemisphere, x = varb, ignore.case = TRUE)
  return(ifelse(SW, -1, 1) * decdg)
}

# References:
# http://stackoverflow.com/questions/14404596/converting-geo-coordinates-from-degree-to-decimal
# https://en.wikipedia.org/wiki/Geographic_coordinate_conversion#Coordinate_format_conversion
# https://en.wikipedia.org/wiki/Decimal_degrees
# tested conversion with http://www.pgc.umn.edu/tools/conversion

# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Examples
x <- read.table(text="DdM     DMS_lat      DMS_lon        DMS
                45°12.123'  45°12'7.38''S  45°12'7.38''W  45°12'7.38
                31°29.17'   31°29'10.2''N  31°29'10.2''E  31°29'10.2 ",
                header=TRUE, stringsAsFactors=FALSE)
x
#          DdM       DMS_lat       DMS_lon        DMS
# 1 45°12.123' 45°12'7.38''S 45°12'7.38''W 45°12'7.38
# 2  31°29.17' 31°29'10.2''N 31°29'10.2''E 31°29'10.2

# case of "degrees decimal minutes" (DdM)
dg2dec(varb=x$DdM, Dg="°", Min="'")
# [1] 45.20205 31.48617

# case of "degrees minutes seconds" (DMS) for latitude 
# with character for Northern (N) and Southern (S) Hemispheres
# use regular expression metacharacter | to indicate logical operation "OR"
dg2dec(varb=x$DMS_lat, Dg="°", Min="'", Sec="''S|N")
# [1] -45.20205  31.48538

# case of "degrees minutes seconds" (DMS) for longitude 
# with character for Eastern (E) and Western (W) Hemispheres
dg2dec(varb=x$DMS_lon, Dg="°", Min="'", Sec="''E|W")
# [1] -45.20205  31.48538

# case of "degrees minutes seconds" (DMS) without seconds symbol. 
# Note that this returns only positive coordinates!
dg2dec(varb=x$DMS, Dg="°", Min="'")
# [1] 45.20205 31.48538

# Sometimes, some regular expression metacharacter 
# could appear in the DMS format (e.g. when reading data from webpages)
# here is a way to deal with that:
test <- "43°37'42\" N" 
# where seconds are identified by (\" N) or (\" W), 
# because \ is a metacharacter in regular expression,
# in R one will have to escape it like \\\\,
# that is why you see below Sec='\\\\" N'
dg2dec(test, Dg='°', Min="'", Sec='\\\\" N')

# Data table examples:
library(data.table)
x.dt <- data.table(x) # transform data frame "x" to data table "x.dt"
# create new column "DMS_dec" by applying dg2dec() function on column "DMS"
x.dt[, DMS_dec := dg2dec(DMS, Dg="°", Min="'")]
x.dt # check results
#           DdM       DMS_lat       DMS_lon        DMS  DMS_dec
# 1: 45°12.123' 45°12'7.38''S 45°12'7.38''W 45°12'7.38 45.20205
# 2:  31°29.17' 31°29'10.2''N 31°29'10.2''E 31°29'10.2 31.48538

Thanks for the code, made life easier. I don't exactly understand what the error is but the code does not transform Seconds data unless I delete the "+ ifelse(dim(DMS)[1] > 2 & !is.na(Sec), " and ", 0)" statement.

Thanks for pointing that out. I think the ifesle might fail because how I set the conditions. I hope to have some time to check that , but meanwhile a colleague happened to tell me about this package https://github.com/ropensci/parzer That might be more useful.

Wow, that package works like a charm, thanks!

parzer package https://github.com/ropensci/parzer, made the task easy.